File System Management

ABSTRACT

Methods, systems, and apparatus, including computer programs encoded on a computer storage medium, for file system management. In one aspect, a method includes receiving a request at a kernel to perform a file operation on a particular file; identifying a flag bit associated with the file; invoking user space file handler to perform particular action when the flag bit is set; determining whether notification that action has been performed by file handler is received; and allowing the file operation on the file when notification is received.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit under 35 U.S.C. §119(e) of thefiling date of U.S. Patent Application No. 61/433,180, for File SystemManagement, which was filed on Jan. 14, 2011, and which is incorporatedhere by reference.

BACKGROUND

This specification relates to file systems.

Requests for particular file operations, for example to read a file, aresent to the operating system kernel (e.g., using a system call). Thekernel manages resources (e.g., system hardware as well as particularinformation such as a database record) available to perform theoperations. The kernel allocates resources for performing the requestedoperation. Conventionally, resources can only be used by a singleprocess at a time. When two processes are running, they can each bewaiting for an event of the other to occur in order to proceed. Forexample, the event that each process is waiting for can be the releaseof a respective recourse currently possessed by the other process.However, since both processes are waiting for the other, neither willcause the event. This results in a deadlock.

SUMMARY

This specification describes technologies relating to file systemmanagement.

In general, one innovative aspect of the subject matter described inthis specification can be embodied in methods that include the actionsof receiving a request at a kernel to perform a file operation on aparticular file; identifying a flag bit associated with the file;invoking user space file handler to perform particular action when theflag bit is set; determining whether notification that action has beenperformed by file handler is received; and allowing the file operationon the file when notification is received. Other embodiments of thisaspect include corresponding systems, apparatus, and computer programsrecorded on computer storage devices, each configured to perform theoperations of the methods.

These and other embodiments can each optionally include one or more ofthe following features. Invoking the user space file handler includesholding the operation and setting a time for the file handler to performthe action without setting any resource locks. Expiration of the settime without a request for further time results in the kernel performingthe file operation. The file operation is a read file operation and thefile hander requests the file data from a network location. The fileoperation is a modify file operation and the file handler copies thefile prior to performing the modify file operation being performed. Themethod further includes identifying i-node information for the file,where the i-node information includes one or more flag bits. The methodfurther includes when the flag bit is set, accessing extended attributeinformation associated with the file to identify special file handlingoperation to be performed by file handler.

Particular embodiments of the subject matter described in thisspecification can be implemented so as to realize one or more of thefollowing advantages. Techniques are provided for invoking user spacehandlers for kernel space events. As a result, file operations are alsointercepted before they are performed. A hold on a request can beinitiated while a user space handler is invoked without setting anylocks. Thus, deadlocks can be avoided. Invoking user space handlersprovides greater flexibility in performing operations than in the kernelspace, for example, initiating network connections to retrieve data.Special file handling can be identified for files by examining i-nodeflags. Using an i-node flag to identify special file handling provides aquick check and reduces input/output (I/O) operations.

The details of one or more embodiments of the subject matter describedin this specification are set forth in the accompanying drawings and thedescription below. Other features, aspects, and advantages of thesubject matter will become apparent from the description, the drawings,and the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of an example system.

FIG. 2 is an example diagram of kernel space interactions.

FIG. 3 is a flow diagram of an example process for performing fileoperations.

FIG. 4 is an example system architecture.

Like reference numbers and designations in the various drawings indicatelike elements.

DETAILED DESCRIPTION

When a file operation request is received at the kernel, for example, asa system call to read a file or to modify a file (write), adetermination is made as to whether additional operations are to beperformed before allowing the requested file operation. If additionaloperations are to be performed, the kernel institutes a hold on the fileoperation, without reserving any resources to the request (e.g., nolocks), and sets a timeout timer. An extended attribute of the fileidentifies special file handling to be performed on the file. Acorresponding user space file handler is requested to perform one ormore actions within the time provided by the timer. When the timerexpires, the hold is released and the requested resources are allocatedfor performing the requested file operation.

FIG. 1 is a block diagram of an example system 100. The system 100includes a user device 102 that can communicate with one or more networklocations 106 through a network 104.

The user device 102 can represent various forms of digital computingdevices, for example, laptops, desktops, workstations, personal digitalassistants, tablet computers, and other mobile devices (e.g.,smartphones, mobile phones). The network 104 can be, for example, alocal area network (LAN), wide area network (WAN), the Internet, or acombination thereof. Communication through the network 104 can beperformed using one or more wired, wireless, or a combination ofconnections. The network locations 106 can be other computing devices,remote storage locations, servers, or other data repositories accessibleby the user device 102.

The user device 102 includes both a kernel space 110 and user space 108.In particular, an operating system typically segregates virtual memoryinto the kernel space 110 and the user space 108. The kernel space 110is reserved for running the kernel, kernel extensions, and devicedrivers. The user space 108 includes the memory area where user modeapplications work. The kernel represents a bridge between applicationsand the actual data processing done at the hardware level. The kernel'sresponsibilities include managing the system's resources. Particularoperations can be performed in the user space that cannot be performedin the kernel space and vice versa. For example, network connections canbe made in user space but not in kernel space.

In response to a request, for example, to perform a file modification ora file open, the kernel can invoke one or more file handlers in the userspace 108 to perform particular actions prior to assigning resources inresponse to the request. The user space handler can perform operationsincluding opening network connections in order to request data from oneor more network locations 106 through the network 104.

FIG. 2 is an example diagram of kernel space interactions 200. A fileoperation request is received by the kernel 204 (e.g., from a user spaceapplication). Based on an analysis of the request, the kernel holds therequest and invokes a user space handler 206 to perform one or moreactions (e.g., according to a token sent to the user space file handler206). In particular, the kernel can read an extended attribute 208associated with the file to identify the particular file operations tobe performed, e.g., prior to allowing the requested file operation toproceed. The kernel does not place a lock on any resources. The kernelfurther can set a timer in which the user space handler 206 has toperform the one or more actions or request additional time to performthe one or more actions.

The user space hander 206 performs the one or more actions and sends thetoken back to the kernel 204 indicating that the actions have beencompleted. Alternatively, the user space handler 206 can send a requestfor more time. The kernel 204 can increase or reset the timer inresponse to the request for more time. Once notified of the completedaction, the kernel 204 releases the hold and assigns resources forperforming the requests file operation.

No locks are placed by the kernel 204. Therefore, if the timer runs outwithout receiving notification or a request for time from the user spacehandler 206, the hold is released by the kernel 204 and resources areallocated for attempting to perform the requested file operation, afterwhich control returns to the application (e.g., to perform furtheroperations based on the requested file operation, for example,presenting file contents based on a requested read operation).

FIG. 3 is a flow diagram of an example process 300 for performing fileoperations. A request to perform a file operation is received (302). Therequest can be a system call to perform various types of fileoperations. For example, a read operation (e.g., to read a file) or awrite operation (e.g., to edit file data). The received request can besent to the kernel in response to user actions, for example, a userselecting a file to open or a user performing an edit on an open file.

File information associated with the request is identified (304). Forexample, in order to perform a read or write of a file, the file data isidentified according to the file's i-node or other catalog record. Thisis a unique identifier for the file that is indexed (e.g., a table ofi-nodes) to file information including the location of blocks of data inwhich the file is stored, time stamps, last accessed, owner of file,permissions, size, and/or creation time. The kernel can load the i-nodefile information into memory. The i-node can also be associated with oneor more flag bits.

A determination is made as to whether a particular flag bit is set(306). This particular flag bit indicates that special file handlingshould be performed. However, as a single bit this does not indicatewhat processing is to be performed on the file. Instead, when the flagbit is set, the kernel reads the extended attribute associated with thefile. The extended attribute is an additional data structure associatedwith the file that describes the special file handling to be performed.Since the i-node (or corresponding catalog record) is loaded into memoryby the kernel, little processing cost is required to check the flag bits(e.g., no additional I/O operations are required). If the flag bit isnot set, the kernel can proceed to respond to the request.

For example, the extended attribute can provide metadata about the typeof file and what special handling is needed to be performed. Forexample, the extended attribute can indicate that the particular file isa data-less file (e.g., the file is only virtually present) and that thecorresponding file data needs to be retrieved before the requestedoperation can be performed (e.g., the file data has to be retrievedbefore the file can be read). In another example, the extended attributecan indicate that the particular file is to be backed up before therequested operation is performed (e.g., to back-up a copy of the fileprior to modifying the file). In some implementations, the extendedattribute identifies a particular user space handler to be invoked(e.g., designed to perform particular operations).

A user space file handler is invoked to perform the one or more actionsindicated by the extended attribute (308). For example, in someimplementations, the file hander has already called into the kernelrequesting work and is waiting to be assigned a token. Alternatively,the specific user space file handler is invoked by the kernel. Thekernel sends the token to the file handler indicating the actions to beperformed with respect to the file. The file hander then initiates athread to perform the requested action. For example, the file handlercan open one or more network connections to a destination in which thefile data is stored. The file data can then be retrieved and associatedwith the existing zero data file.

In addition to invoking the user space file handler, the kernel holdsthe requested operation and sets a timer (310). The timer provides atime limit for the user space file handler to perform the indicatedoperation. While the timer holds the requested operation, no resourcesare locked out by the kernel. Thus, no deadlocks can occur from therequest (e.g., from a resource request made by the user space filehander). As a result, other requests for resources can be made (e.g., aspart of other processes) while the user space file handler is performingthe one or more actions.

A determination is made as to whether the timer has expired beforereceiving a response from the user space file handler (312). If aresponse is not received, the hold is released (314) and the requestedoperation proceeds (322). For example, the kernel can allocate thenecessary resources for performing the requested operation. Theoperation may or may not be able to be performed. For example, if therequested file operation is to read a file that is not present, then anerror will result from the attempted read operation. However, if therequested file operation is a file modification to an open file, themodification can be performed.

If a response is received from the user space file handler (316) beforethe timer has expired, a determination is made as to whether or not theaction or actions have been completed (318). If the action or actionshave not been completed, a request for more time can be sent by the userspace file handler. The timer is extended or reset (320). The processthen returns to the determination of whether a response is receivedbefore the timer expires (312). The hold on the file operation ismaintained until the timer again expires.

If the action has been completed, the hold is released and the requestedfile operation proceeds (322). The user space file handler can indicatethat the action has been complete, for example, by returning thereceived token to the kernel or other notification. The file operationcan proceed, for example, by the kernel assigning resources to performthe requested operation. For example, when the request is a read filerequest, the resources are allocated for reading the file into memory.The file data can then, for example, be presented to the user (e.g., asa document).

Specific examples are provided below to further illustrate the process300.

In some implementations, data-less data items are created that presentthe appearance of data items that are not located on the particular userdevice. For example, items being synchronized or downloaded from otherlocations (e.g., other user devices or network locations) may not havethe data to be synchronized or downloaded stored locally. Differentcriteria can be used to determine whether or not to download the data.For example, the size of the file, the current transfer rate, the typeof data, etc. As such, the data can be stored elsewhere and retrievedwhen requested by the user. In order to present the appearance of thedata, virtual data items can be generated. For example, files can appearto be present on the user device (e.g., as a displayed icon in aparticular directory), but the file data is not physically present.

Similarly, in another implementation, data items can be removed from theuser device and stored in a network location while retaining the virtualitem representation. Again, different criteria can be used to determinewhether or not to remove a data item from physically being located onthe user device. For example, data items that are accessed infrequentlycan be removed to save space. Additionally, data items can be removedbased on a percentage or amount of free space remaining on the device tostore data (e.g., when a threshold storage level is reached).

These virtual data items can appear as any other file, directory, orother data on the file system or can be identified as virtual files(e.g., with a particular badging of an associated icon).

When a user requests one of these virtual data items, for example afile, the data must be retrieved before the file read is attemptedotherwise an error will result. Consequently, the read request the flagbit indicates that the file is an empty file and that the file data mustbe retrieved. The read request is held by the kernel while a particularuser space file handler is requested in order to retrieve the associatedfile data.

The user space file handler performs the network connections to thestorage location of the file data (e.g., at a location indicated in theextended attribute data). The retrieved file data is then associatedwith the data-less data file such that the file now has the filecontents (e.g., the data-less file can be modified or replaced). If theuser space file handler fails to perform the actions, for example,because the network connections are unavailable, a time out will resultin the hold being released by the kernel. However, since there is nodata to be read, an error will result. This read error can be presentedto the user indicating that the action is unable to be performed at thepresent time.

Once the file data is retrieved, the user space file handler indicatesto the kernel that the actions have been performed and the kernelreleases the hold on the read operation. The file is then read, forexample, for presentation to the user by an application associated withthe file.

In some other implementations, changes to data items are tracked andperiodically stored prior to any changes being made. For example, amodification to a data item can trigger a copy of the data item (e.g., aversion of the data item) being stored prior to performing themodification. The user device can maintain a collection of versions ofthe data item for later retrieval by the user.

Thus, in order to store copies of the file prior to modification, thefile modification request has to be intercepted and the copy performedprior to allowing the modification operation to proceed.

When a request to perform a file modification operation is received bythe kernel, the kernel identifies the flag bit as set. The extendedattribute indicates that the file is one to be tracked and that a copyshould be made before a file modification operation. In someimplementations, this is variable depending on specific criteria. Forexample, only one copy can be made for a given time period (e.g., 1hour). Thus, a copy is made of the file prior to the first modificationof the file.

However, any subsequent modifications to the same file within that timeperiod are allowed to proceed without copying the file. Thus, a filethat changes 10 times within an hour will only be copied the first timeafter the start of the specified time period. Later requests to modifythe file within the specified time period will not trigger the specialfile handling prior to performing the requested operation.

In some other implementations, when a request to perform a filemodification operation is received, a determination is made as towhether a prior operation has been performed on the file within aspecified time period. For example, the time of the requestedmodification can be compared to operations performed on the file withina current fixed window of time. For example, the specified time periodcan be one of a sequence of fixed time windows (e.g., every hour blockof time can form a specified time period regardless of when a prior filemodification operation occurred). If there has not been a prioroperation on the file within a current time window (or less than aspecified number of modifications), the file is copied prior to themodification operation being performed. In some implementations, inaddition to the determination with respect to time, the type ofrequested modification is considered. For example, an edit operation cantrigger the copy being made while a name change may not.

If there has been a prior operation on the file within the current timewindow, the requested modification can be allowed within performing anyintervening action (e.g., copying the unmodified version of the file).For example, when only one action (e.g., copy file) per time window isperformed prior to a requested modification operation on a file.

If the file is to be copied prior to the modification operation, theuser space file handler is called to perform the file copy operation.When the kernel is notified that the operation is complete, the hold onthe operation is removed and the file is allowed to be modified.

If a timeout occurs without a response from the user space file handler,the hold is released and the modification is allowed to occur. Thus, nopre-modified version will be stored.

In some other implementations, files can have a flag bit set when thefile is to be tracked. For example, when versions of a file are storedas described above, the versions can be tracked to make sure all theversions are linked together. In another example, if a request for aname change is received for a tracked file the name change is held whileperforming a link between the new file and the earlier versions.

FIG. 4 illustrates an example architecture of a system 400. The systemarchitecture 400 is capable of performing operations for performing filesystem management. The architecture 400 includes one or more processors402 (e.g., IBM PowerPC, Intel Pentium 4, etc.), one or more displaydevices 404 (e.g., CRT, LCD), graphics processing units 406 (e.g.,NVIDIA GeForce, etc.), a network interface 408 (e.g., Ethernet,FireWire, USB, etc.), input devices 410 (e.g., keyboard, mouse, etc.),and one or more computer-readable mediums 412. These components exchangecommunications and data using one or more buses 414 (e.g., EISA, PCI,PCI Express, etc.).

The term “computer-readable medium” refers to any medium thatparticipates in providing instructions to a processor 402 for execution.The computer-readable medium 412 further includes an operating system416 (e.g., Mac OS®, Windows®, Linux, etc.), a network communicationmodule 418, user space file handlers 422, and other applications 424.

The operating system 416 can be multi-user, multiprocessing,multitasking, multithreading, real-time and the like. The operatingsystem 416 performs basic tasks, including but not limited to:recognizing input from input devices 410; sending output to displaydevices 404; keeping track of files and directories on computer-readablemediums 412 (e.g., memory or a storage device); controlling peripheraldevices (e.g., disk drives, printers, etc.); and managing traffic on theone or more buses 414. The operating system 416 also includes kernelfunctions. The network communications module 418 includes variouscomponents for establishing and maintaining network connections (e.g.,software for implementing communication protocols, such as TCP/IP, HTTP,Ethernet, etc.).

The operating system 416, particularly the kernel, and the user spacefile handlers 422 provide various software components for performing thevarious functions for holding file operation requests at the kernel andinvoking a user space handler to perform one or more actions beforereleasing or timing out the hold on the requested operation as describedwith respect to FIGS. 1-3.

Embodiments of the subject matter and the operations described in thisspecification can be implemented in digital electronic circuitry, or incomputer software, firmware, or hardware, including the structuresdisclosed in this specification and their structural equivalents, or incombinations of one or more of them. Embodiments of the subject matterdescribed in this specification can be implemented as one or morecomputer programs, i.e., one or more modules of computer programinstructions, encoded on computer storage medium for execution by, or tocontrol the operation of, data processing apparatus. Alternatively or inaddition, the program instructions can be encoded on anartificially-generated propagated signal, e.g., a machine-generatedelectrical, optical, or electromagnetic signal, that is generated toencode information for transmission to suitable receiver apparatus forexecution by a data processing apparatus. A computer storage medium canbe, or be included in, a computer-readable storage device, acomputer-readable storage substrate, a random or serial access memoryarray or device, or a combination of one or more of them. Moreover,while a computer storage medium is not a propagated signal, a computerstorage medium can be a source or destination of computer programinstructions encoded in an artificially-generated propagated signal. Thecomputer storage medium can also be, or be included in, one or moreseparate physical components or media (e.g., multiple CDs, disks, orother storage devices).

The operations described in this specification can be implemented asoperations performed by a data processing apparatus on data stored onone or more computer-readable storage devices or received from othersources.

The term “data processing apparatus” encompasses all kinds of apparatus,devices, and machines for processing data, including by way of example aprogrammable processor, a computer, a system on a chip, or multipleones, or combinations, of the foregoing The apparatus can includespecial purpose logic circuitry, e.g., an FPGA (field programmable gatearray) or an ASIC (application-specific integrated circuit). Theapparatus can also include, in addition to hardware, code that createsan execution environment for the computer program in question, e.g.,code that constitutes processor firmware, a protocol stack, a databasemanagement system, an operating system, a cross-platform runtimeenvironment, a virtual machine, or a combination of one or more of them.The apparatus and execution environment can realize various differentcomputing model infrastructures, such as web services, distributedcomputing and grid computing infrastructures.

A computer program (also known as a program, software, softwareapplication, script, or code) can be written in any form of programminglanguage, including compiled or interpreted languages, declarative orprocedural languages, and it can be deployed in any form, including as astand-alone program or as a module, component, subroutine, object, orother unit suitable for use in a computing environment. A computerprogram may, but need not, correspond to a file in a file system. Aprogram can be stored in a portion of a file that holds other programsor data (e.g., one or more scripts stored in a markup languagedocument), in a single file dedicated to the program in question, or inmultiple coordinated files (e.g., files that store one or more modules,sub-programs, or portions of code). A computer program can be deployedto be executed on one computer or on multiple computers that are locatedat one site or distributed across multiple sites and interconnected by acommunication network.

The processes and logic flows described in this specification can beperformed by one or more programmable processors executing one or morecomputer programs to perform actions by operating on input data andgenerating output. The processes and logic flows can also be performedby, and apparatus can also be implemented as, special purpose logiccircuitry, e.g., an FPGA (field programmable gate array) or an ASIC(application-specific integrated circuit).

Processors suitable for the execution of a computer program include, byway of example, both general and special purpose microprocessors, andany one or more processors of any kind of digital computer. Generally, aprocessor will receive instructions and data from a read-only memory ora random access memory or both. The essential elements of a computer area processor for performing actions in accordance with instructions andone or more memory devices for storing instructions and data. Generally,a computer will also include, or be operatively coupled to receive datafrom or transfer data to, or both, one or more mass storage devices forstoring data, e.g., magnetic, magneto-optical disks, or optical disks.However, a computer need not have such devices. Moreover, a computer canbe embedded in another device, e.g., a mobile telephone, a personaldigital assistant (PDA), a mobile audio or video player, a game console,a Global Positioning System (GPS) receiver, or a portable storage device(e.g., a universal serial bus (USB) flash drive), to name just a few.Devices suitable for storing computer program instructions and datainclude all forms of non-volatile memory, media and memory devices,including by way of example semiconductor memory devices, e.g., EPROM,EEPROM, and flash memory devices; magnetic disks, e.g., internal harddisks or removable disks; magneto-optical disks; and CD-ROM and DVD-ROMdisks. The processor and the memory can be supplemented by, orincorporated in, special purpose logic circuitry.

To provide for interaction with a user, embodiments of the subjectmatter described in this specification can be implemented on a computerhaving a display device, e.g., a CRT (cathode ray tube) or LCD (liquidcrystal display) monitor, for displaying information to the user and akeyboard and a pointing device, e.g., a mouse or a trackball, by whichthe user can provide input to the computer. Other kinds of devices canbe used to provide for interaction with a user as well; for example,feedback provided to the user can be any form of sensory feedback, e.g.,visual feedback, auditory feedback, or tactile feedback; and input fromthe user can be received in any form, including acoustic, speech, ortactile input. In addition, a computer can interact with a user bysending documents to and receiving documents from a device that is usedby the user; for example, by sending web pages to a web browser on auser's client device in response to requests received from the webbrowser.

Embodiments of the subject matter described in this specification can beimplemented in a computing system that includes a back-end component,e.g., as a data server, or that includes a middleware component, e.g.,an application server, or that includes a front-end component, e.g., aclient computer having a graphical user interface or a Web browserthrough which a user can interact with an implementation of the subjectmatter described in this specification, or any combination of one ormore such back-end, middleware, or front-end components. The componentsof the system can be interconnected by any form or medium of digitaldata communication, e.g., a communication network. Examples ofcommunication networks include a local area network (“LAN”) and a widearea network (“WAN”), an inter-network (e.g., the Internet), andpeer-to-peer networks (e.g., ad hoc peer-to-peer networks).

The computing system can include clients and servers. A client andserver are generally remote from each other and typically interactthrough a communication network. The relationship of client and serverarises by virtue of computer programs running on the respectivecomputers and having a client-server relationship to each other. In someembodiments, a server transmits data (e.g., an HTML page) to a clientdevice (e.g., for purposes of displaying data to and receiving userinput from a user interacting with the client device). Data generated atthe client device (e.g., a result of the user interaction) can bereceived from the client device at the server.

While this specification contains many specific implementation details,these should not be construed as limitations on the scope of anyinventions or of what may be claimed, but rather as descriptions offeatures specific to particular embodiments of particular inventions.Certain features that are described in this specification in the contextof separate embodiments can also be implemented in combination in asingle embodiment. Conversely, various features that are described inthe context of a single embodiment can also be implemented in multipleembodiments separately or in any suitable subcombination. Moreover,although features may be described above as acting in certaincombinations and even initially claimed as such, one or more featuresfrom a claimed combination can in some cases be excised from thecombination, and the claimed combination may be directed to asubcombination or variation of a subcombination.

Similarly, while operations are depicted in the drawings in a particularorder, this should not be understood as requiring that such operationsbe performed in the particular order shown or in sequential order, orthat all illustrated operations be performed, to achieve desirableresults. In certain circumstances, multitasking and parallel processingmay be advantageous. Moreover, the separation of various systemcomponents in the embodiments described above should not be understoodas requiring such separation in all embodiments, and it should beunderstood that the described program components and systems cangenerally be integrated together in a single software product orpackaged into multiple software products.

Thus, particular embodiments of the subject matter have been described.Other embodiments are within the scope of the following claims. In somecases, the actions recited in the claims can be performed in a differentorder and still achieve desirable results. In addition, the processesdepicted in the accompanying figures do not necessarily require theparticular order shown, or sequential order, to achieve desirableresults. In certain implementations, multitasking and parallelprocessing may be advantageous.

What is claimed is: 1-24. (canceled)
 25. A method comprising: receivinga request at a kernel to perform a file operation on a particular file;determining at the kernel that a flag bit associated with the file isset; invoking by the kernel a user space file handler to perform aparticular action in user space when the flag bit is set; determining atthe kernel whether a notification has been received that the particularaction has been performed by the file handler in user space; andallowing, at the kernel, the file operation on the particular file whenthe notification is received.
 26. The method of claim 25, whereininvoking the user space file handler includes holding the operation andsetting a time for the file handler to perform the action withoutsetting any resource locks.
 27. The method of claim 26, whereinexpiration of the set time without a request for further time results inthe kernel performing the file operation.
 28. The method of claim 25,wherein the file operation is a read file operation and the file handerrequests the file data from a network location.
 29. The method of claim25, wherein the file operation is a modify file operation and the filehandler copies the file prior to performing the modify file operationbeing performed.
 30. The method of claim 25, further comprising:identifying i-node information for the file, wherein the i-nodeinformation includes one or more flag bits.
 31. The method of claim 30,further comprising: when the flag bit is set, accessing extendedattribute information associated with the file to identify special filehandling operation to be performed by file handler.
 32. The method ofclaim 31, wherein the extended attribute indicates that the particularfile is to be backed up before the requested operation is performed. 33.The method of claim 31, wherein the extended attribute identifies aparticular user space handler to be invoked, the particular user spacehandler being designated to perform particular operations.
 34. Themethod of claim 25, wherein the flag bit associated with the fileindicates that the file is an empty file and that the file data must beretrieved from a network location.
 35. A system comprising: one or morecomputing devices operable to perform operations comprising: receiving arequest at a kernel to perform a file operation on a particular file;determining at the kernel that a flag bit associated with the file isset; invoking by the kernel a user space file handler to perform aparticular action in user space when the flag bit is set; determining atthe kernel whether a notification has been received that the particularaction has been performed by the file handler in user space; andallowing, at the kernel, the file operation on the particular file whenthe notification is received.
 36. The system of claim 35, whereininvoking the user space file handler includes holding the operation andsetting a time for the file handler to perform the action withoutsetting any resource locks.
 37. The system of claim 36, whereinexpiration of the set time without a request for further time results inthe kernel performing the file operation.
 38. The system of claim 35,wherein the file operation is a read file operation and the file handerrequests the file data from a network location.
 39. The system of claim35, wherein the file operation is a modify file operation and the filehandler copies the file prior to performing the modify file operationbeing performed.
 40. The system of claim 35, further operable to performoperations comprising: identifying i-node information for the file,wherein the i-node information includes one or more flag bits.
 41. Thesystem of claim 40, further operable to perform operations comprising:when the flag bit is set, accessing extended attribute informationassociated with the file to identify special file handling operation tobe performed by file handler.
 42. The system of claim 35, wherein theextended attribute indicates that the particular file is to be backed upbefore the requested operation is performed.
 43. The system of claim 35,wherein the extended attribute identifies a particular user spacehandler to be invoked, the particular user space handler beingdesignated to perform particular operations.
 44. The system of claim 35,wherein the flag bit associated with the file indicates that the file isan empty file and that the file data must be retrieved from a networklocation.
 45. A non-transitory computer storage medium encoded with acomputer program, the program comprising instructions that when executedby data processing apparatus cause the data processing apparatus toperform operations comprising: receiving a request at a kernel toperform a file operation on a particular file; determining at the kernelthat a flag bit associated with the file is set; invoking by the kernela user space file handler to perform a particular action in user spacewhen the flag bit is set; determining at the kernel whether anotification has been received that the particular action has beenperformed by the file handler in user space; and allowing, at thekernel, the file operation on the particular file when the notificationis received.
 46. The non-transitory computer storage medium of claim 45,wherein invoking the user space file handler includes holding theoperation and setting a time for the file handler to perform the actionwithout setting any resource locks.
 47. The non-transitory computerstorage medium of claim 46, wherein expiration of the set time without arequest for further time results in the kernel performing the fileoperation.
 48. The non-transitory computer storage medium of claim 45,wherein the file operation is a read file operation and the file handerrequests the file data from a network location.
 49. The non-transitorycomputer storage medium of claim 45, wherein the file operation is amodify file operation and the file handler copies the file prior toperforming the modify file operation being performed.
 50. Thenon-transitory computer storage medium of claim 45, further operable tocause the data processing apparatus to perform operations comprising:identifying i-node information for the file, wherein the i-nodeinformation includes one or more flag bits.
 51. The non-transitorycomputer storage medium of claim 45, further operable to cause the dataprocessing apparatus to perform operations comprising: when the flag bitis set, accessing extended attribute information associated with thefile to identify special file handling operation to be performed by filehandler.
 52. The non-transitory computer storage medium of claim 45,wherein the extended attribute indicates that the particular file is tobe backed up before the requested operation is performed.
 53. Thenon-transitory computer storage medium of claim 45, wherein the extendedattribute identifies a particular user space handler to be invoked, theparticular user space handler being designated to perform particularoperations.
 54. The non-transitory computer storage medium of claim 45,wherein the flag bit associated with the file indicates that the file isan empty file and that the file data must be retrieved from a networklocation.
 55. A method comprising: receiving a request at a kernel toperform a file operation on a particular file; determining at the kernelwhether a prior operation has been performed on the file within aspecified time period; invoking by the kernel a user space file handlerto perform a particular action in user space when no prior operation hasbeen performed within the time period; determining at the kernel whethera notification has been received that the particular action has beenperformed by the file handler in user space; and allowing, at thekernel, the file operation on the particular file when the notificationis received.
 56. The method of claim 55, wherein the request is a filemodification and the particular action is to perform a copy of the file.57. The method of claim 55, wherein the specified time period is one ofa sequence of fixed time windows, wherein the action is performed on thefile once during each window.